Semiconductor materials containing indium and phosphorus are important, for example, in the fabrication of semiconductor lasers for operation at wavelengths in the infra-red, especially near to 1.3 and 1.55 .mu.m, at which wavelengths silica optical fibres usually have loss minima, the minimum at 1.55 .mu.m being the deeper one. Operation near these wavelengths is extremely attractive for telecommunication purposes.
Both for telecommunications and other purposes it is often desirable that the laser power should be concentrated into a very narrow frequency range. In the case of telecommunications systems with silica fibres, this is especially important for operation near 1.55 .mu.m where the materials dispersion in the fibre is usually much greater than near 1.3 .mu.m.
In one of the simplest semiconductor laser designs (the Fabry-Perot type), the laser usually operates, undesirably for such purposes, in a plurality of longitudinal modes of differing wavelength. In addition, the precise wavelengths of the modes depend on the dimensions of the laser cavity and this restricts one's freedom to use such lasers in integrated optics structures (since in these structures the effective dimensions of the laser cavity are a function of the other devices in the integrated structure).
Longitudinal mode control can be achieved by means of a diffraction grating. One laser structure incorporating a diffraction grating is known as the distributed feedback (DFB) laser (see G. H. B. Thompson, Semiconductor Lasers, Wiley, 1980). A DFB laser operating at 1.53 .mu.m has been described by K. Sakai, K. Utaka, S. Akiba, and Y. Matsushima (IEEE J. Quantum Electronics, QE-18, no. 8, pages 1272-1278, August 1982). In constructing their laser they made a first order diffraction grating with a period of 2365 .ANG. (0.2365 .mu.m) on the surface of doped InP by holographic techniques and chemical etching. The corrugation depth thus achieved was typically 1000 .ANG. (0.1 .mu.m), but the subsequent growth of a doped quaternary layer (i.e., a layer containing Ga, In, As and P plus dopant) reduced the corrugation depth of 200-500 .ANG. (0.02-0.05 .mu.m). They attributed this to dissolution of the grating in the melt used for growing the quaternary layer. (This technique of growth is known as liquid phase epitaxy of LPE).
The final depth of the DFB corrugations is one of the most important parameters of the device. The reduction of the corrugation depth in LPE has an adverse effect on laser efficiency and in particular raises the threshold current of the laser. High threshold currents make for heating of the laser in use and consequent control difficulties and for low upper working temperatures.
In general, one can say that the spectral purity and threshold current of the DFB lasers are strongly dependent on the precise positioning and cross-section profile of the integral diffraction grating, and that close control of the manufacturing processes incorporating these sub-micron period corrugations into the laser structure is highly desirable.